Write to temp file before saving data (#238)

* Create temp file before saving data

On the event of full disk, we may encounter the case where the
destination file get replaced by an empty file as the
ioutil.WriteFile truncates the destination before write.

* Close the tempFile instance before moving it

* Blacklist Wireguard interfaces for ICE checks

util/file.go

@@ -12,7 +12,7 @@ import (
 // The output JSON is pretty-formatted
 func WriteJson(file string, obj interface{}) error {
 
-	configDir := filepath.Dir(file)
+	configDir, configFileName := filepath.Split(file)
 	err := os.MkdirAll(configDir, 0750)
 	if err != nil {
 		return err
@@ -24,7 +24,31 @@ func WriteJson(file string, obj interface{}) error {
 		return err
 	}
 
-	err = ioutil.WriteFile(file, bs, 0600)
+	tempFile, err := ioutil.TempFile(configDir, ".*"+configFileName)
+	if err != nil {
+		return err
+	}
+
+	tempFileName := tempFile.Name()
+	// closing file ops as windows doesn't allow to move it
+	err = tempFile.Close()
+	if err != nil {
+		return err
+	}
+
+	defer func() {
+		_, err = os.Stat(tempFileName)
+		if err == nil {
+			os.Remove(tempFileName)
+		}
+	}()
+
+	err = ioutil.WriteFile(tempFileName, bs, 0600)
+	if err != nil {
+		return err
+	}
+
+	err = os.Rename(tempFileName, file)
 	if err != nil {
 		return err
 	}